A gas giant is a giant planet composed mainly of hydrogen and helium. Jupiter and Saturn are the Solar System's gas giants. The term "gas giant" was originally synonymous with "giant planet", but in the 1990s it became known that Uranus and Neptune are really a distinct class of giant planet, being composed mainly of heavier volatile substances (which are referred to as 'ices'), and therefore increasingly referred to as ice giants.
Jupiter and Saturn consist mostly of hydrogen and helium, with heavier elements making up between 3 and 13 percent of the mass. Their structures are thought to consist of an outer layer of molecular hydrogen, surrounding a layer of liquid metallic hydrogen, with probably a molten core with a rocky composition. The outermost portion of the hydrogen atmosphere is characterized by many layers of visible clouds that are mostly composed of water and ammonia. The layer of metallic hydrogen makes up the bulk of each planet, and is referred to as "metallic" because the very large pressure turns hydrogen into an electrical conductor. The core is thought to consist of heavier elements at such high temperatures (20,000 K) and pressures that their properties are poorly understood.
- The Interior of Jupiter, Guillot et al., in Jupiter: The Planet, Satellites and Magnetosphere, Bagenal et al., editors, Cambridge University Press, 2004